Use codex-linux-sandbox in unified exec (#6480)

Unified exec isn't working on Linux because we don't provide the correct
arg0.

The library we use for pty management doesn't allow setting arg0
separately from executable. Use the same aliasing strategy we use for
`apply_patch` for `codex-linux-sandbox`.

Use `#[ctor]` hack to dispatch codex-linux-sandbox calls.


Addresses https://github.com/openai/codex/issues/6450

codex-rs/core/tests/suite/unified_exec.rs

@@ -66,7 +66,7 @@ fn parse_unified_exec_output(raw: &str) -> Result<ParsedUnifiedExecOutput> {
     let cleaned = raw.trim_matches('\r');
     let captures = regex
         .captures(cleaned)
-        .ok_or_else(|| anyhow::anyhow!("missing Output section in unified exec output"))?;
+        .ok_or_else(|| anyhow::anyhow!("missing Output section in unified exec output {raw}"))?;
 
     let chunk_id = captures
         .name("chunk_id")
@@ -1368,6 +1368,8 @@ async fn unified_exec_timeout_and_followup_poll() -> Result<()> {
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+// Skipped on arm because the ctor logic to handle arg0 doesn't work on ARM
+#[cfg(not(target_arch = "arm"))]
 async fn unified_exec_formats_large_output_summary() -> Result<()> {
     skip_if_no_network!(Ok(()));
     skip_if_sandbox!(Ok(()));
@@ -1451,3 +1453,75 @@ PY
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn unified_exec_runs_under_sandbox() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+    skip_if_sandbox!(Ok(()));
+
+    let server = start_mock_server().await;
+
+    let mut builder = test_codex().with_config(|config| {
+        config.features.enable(Feature::UnifiedExec);
+    });
+    let TestCodex {
+        codex,
+        cwd,
+        session_configured,
+        ..
+    } = builder.build(&server).await?;
+
+    let call_id = "uexec";
+    let args = serde_json::json!({
+        "cmd": "echo 'hello'",
+        "yield_time_ms": 500,
+    });
+
+    let responses = vec![
+        sse(vec![
+            ev_response_created("resp-1"),
+            ev_function_call(call_id, "exec_command", &serde_json::to_string(&args)?),
+            ev_completed("resp-1"),
+        ]),
+        sse(vec![
+            ev_assistant_message("msg-1", "done"),
+            ev_completed("resp-2"),
+        ]),
+    ];
+    mount_sse_sequence(&server, responses).await;
+
+    let session_model = session_configured.model.clone();
+
+    codex
+        .submit(Op::UserTurn {
+            items: vec![UserInput::Text {
+                text: "summarize large output".into(),
+            }],
+            final_output_json_schema: None,
+            cwd: cwd.path().to_path_buf(),
+            approval_policy: AskForApproval::Never,
+            // Important!
+            sandbox_policy: SandboxPolicy::ReadOnly,
+            model: session_model,
+            effort: None,
+            summary: ReasoningSummary::Auto,
+        })
+        .await?;
+
+    wait_for_event(&codex, |event| matches!(event, EventMsg::TaskComplete(_))).await;
+
+    let requests = server.received_requests().await.expect("recorded requests");
+    assert!(!requests.is_empty(), "expected at least one POST request");
+
+    let bodies = requests
+        .iter()
+        .map(|req| req.body_json::<Value>().expect("request json"))
+        .collect::<Vec<_>>();
+
+    let outputs = collect_tool_outputs(&bodies)?;
+    let output = outputs.get(call_id).expect("missing output");
+
+    assert_regex_match("hello[\r\n]+", &output.output);
+
+    Ok(())
+}